Various substrates, such as metals, glass and plastics, require the application of a primer before they can be coated with a conventional paint or other coating formulation. Such primer compositions have often relied on one or more organosilane compounds to promote the adhesion between substrate and coating. Thus, for example, Finzel, in U.S. Pat. No. 4,654,236, teaches a method for priming coil coated substrates with a solution of a tetraorganosilane and a tetraalkyltitanate. The primed surfaces can be subsequently painted to provide surfaces which are as durable as the original coil coatings. Similar organosilanes can also be employed in various compositions for purposes other than priming substrates. For example, U.S. Pat. No. 4,846,886 to Fey et al. discloses compositions wherein an improved water repellent composition for porous substrates is formed from an alkylalkoxysilane, a carrier, a water beading agent and a metal salt catalyst.
Of concern herein is the particular difficulty of painting or overcoating a silicone substrate. This operation is particularly troublesome since silicone and silicone polymer substrates characteristically exhibit very low surface energies. They are thus better known in the art for their abhesive nature and typically tend to release most coating compositions applied thereto. Since it is often desirable to paint or otherwise overcoat a silicone substrate (e.g., for decorative purposes. abrasion/erosion resistance, printing, marking) many attempts have been made over the years to develop satisfactory primers for this application. The following disclosures represent the relevant art of this type known to applicant.
Young, in U.S. Pat. No. 3,671,483, discovered that a solution containing a tetraalkyltitanate and a particular silane which has at least one group of the formula --CH.sub.2 CH.sub.2 --Si(OOCCH.sub.3).sub.3 in its molecule is useful as a primer in adhering a silicone rubber to epoxy and polyurethane.
In U.S. Pat. No. 3,677,998, Young also teaches that a solution of an organotitanate, tetra(methoxyethoxy)silane, a methylsiloxane resin and either acetic acid or an organotriacetoxysilane is useful as a primer in adhering a silicone rubber to a metal surface.
Hirai et al., in U.S. Pat. No. 4,598,134, teach a primer composition for heat curable silicone rubber consisting essentially of at least one epoxy-functional organosilicon compound, a trialkoxysilane having either two silicon-bonded hydrogens or two unsaturated groups or a partial hydrolyzate of said trialkoxysilane, and an organotitanate ester.
Saito et al., in U.S. Pat. No. 4,681,636 disclose a primer composition particularly suitable for bonding fluorosilicone rubber to a substrate. This primer comprises a solution of a t-butyl peroxy-functional silane, a reactive organosilicon compound having at least two alkoxy groups and an organotitanate ester. In another disclosure to Saito et al., U.S. Pat. No. 4,749,741, primers suitable for promoting adhesion between silicone rubber and plastics or metals are taught. These primers are similar to their previous systems and comprise a solution of an unsaturated organotitanate ester and either an organosilicon compound having at least two alkoxy groups or an organohydrogensiloxane (or silane).
In U.S. Pat. No. 4,704,419, Fukayama et al. teach primer compositions which improve adhesion of paints to cured silicone rubber and comprise a solution of an amine-functional dialkoxysilane and a liquid hydroxyl terminated polyorganosiloxane, optionally further comprising a catalyst.
Meddaugh, in U.S. Pat. No. 4,923,946, discloses a primer for moisture-cured silicone sealants which imparts paintable surfaces thereto. The primers disclosed contain a solution of a hydrolyzate of an aminoalkyl-substituted trialkoxysilane and a carboxylic acid salt of tin.
A method for bonding to a silicone rubber is disclosed in Japanese Kokai Patent No. 59[1984]-48768 to Toray Industries, Inc. Here, a waterless lithographic plate having a photosensitive layer, an adhesive layer and a silicone rubber layer is described wherein the adhesive layer contains an organic titanate. Printing plates of this invention are stated have a strong adhesion between the photosensitive layer and the silicone rubber layer.
In addition to the above described art, the following United States patents teach various primer compositions which promote adhesion to silicone surfaces: 4,431,472 to Hohl et al., 4,122,127 to Mikami et al., 4,534,815 to Hamada et al., 4,546,018 to Ryuzo et al., 4,133,938 to Bingham, 4,147,685 to Smith, 4,332,844 to Hamada et al., 3,619,256 to Pepe et al., 4,808,483 to Nakasuji et al., 4,436,787 to Mikami et al. and 4,401,500 to Hamada et al.